Knowing where you are can help diagnose problems. You mention it is now hot so assuming somewhere north of the equator.
There are a number of reasons for bonsai leaves turning brow.
1. Not enough water. Dawn redwood are very thirsty. Mine dawn redwoods often sit in trays of water through the hotter months. Trees that have not been repotted for a few years tend to get root bound. All those roots fill the spaces where water should sit. I find that every year my pots get more and more difficult to water properly and often require a soak each week. Judging from teh roots on the surface I'd say it has been some years since this one was repotted.
2. Sunburn. Noting that you have only had the tree for a few weeks so it has had a change of situation. Leaves that are adapted to shade often cannot cope with stronger sun suddenly. Do you know what conditions it had before? Are conditions at your place more or less sunny or afternoon sun instead of mornings? Leaves can adapt but it usually takes several weeks of slow adjustment to adjust to changed sun levels so we need to transition trees gradually if moving to stronger/ more sun area.
3. Overwatering. Unlikely to affect Dawn Redwood, but keeping the roots saturated can set up root rot which slowly kills the tree. Unlikely in this case because it usually takes a month or more to develop.
4. Autumn. Dawn Redwood is deciduous. At the end of Summer the leaves will naturally turn brown and fall off for winter. I'm assuming it's a bit early for that change but we occasionally see trees change early when under some stress from water, sun, etc.
